1. Field of the Invention
The present invention relates to an ink jet printing apparatus, and more specifically, to a recovery operation performed for a printing head in a printing apparatus of ink jet method for forming an image by ejecting ink to a printing medium.
2. Description of the Related Art
An ink jet printing method is a method that forms an image by ejecting ink (hereinafter referred to also as a printing liquid) of a single color or a plurality of colors provided corresponding to full color printing, onto a printing medium of various kinds, such as paper, a cloth, a nonwoven fabric, OHP transparencies and other types of plastic film, and the like. As an ink jet printing apparatus employing the ink jet printing method, there is available a type of ink jet printing apparatus. This type of ink jet printing apparatus is provided with a carriage mounted with printing means (a printing head) and an ink tank serving as an ink storage portion and moved for scanning in a predetermined main scanning direction with respect to the printing medium, transport means for transporting the printing medium in a direction different from the main scanning direction (that is, a sub-scanning direction), and control means for controlling the carriage and the transport means. As the printing head is made to serially scan in the main scanning direction, ink is ejected from a plurality of ink ejection ports provided in the printing head. Meanwhile, after the serial scan motion, the printing medium is transported a predetermined amount (for example, a printing width of one serial scan motion corresponding to an ejection port arrangement range), thereby accomplishing printing serially on the printing medium. The ink jet printing method as described above employs what is called a drop on-demand method, which ejects ink directly onto the printing medium in accordance with a printing signal, is widely used as an easy and low-cost printing method.
The ink jet printing apparatus is generally comprises an ink jet printing head provided with nozzles disposed at a pitch of 1/300 inches, 1/600 inches, or the like. During a time of a non-printing period, that is, while the ink jet printing head is in a standby state, the printing liquid evaporates from the ejection port at a leading edge of the nozzle and, as a result, viscosity of the printing liquid at and around the ejection port increases. This results in an ejection failure occurring in a printed image in the beginnings of a printing operation initiated next. Specific problems include part of ink dots not formed, a deviation produced in a position a printing liquid droplet lands at, and the like. This results in a blurred or incorrect printed image, or the like, occurring.
Approaches to be described below have been taken to solve these problems. Specifically, a cap made generally of a rubber is contacted with a surface (hereinafter referred to as an “ejection port surface”) on which the leading edge of nozzles, that is, the ejection ports are arranged, to prevent the printing liquid from evaporating through the ejection ports. In addition, means for performing ejecting the printing liquid irrelevant to printing at a location other than a printing portion for a predetermined period of time for example at a start of printing (hereinafter referred to as a “preliminary ejection”) are provided. With the preliminary ejection, the printing liquid at and around the ejection ports, the viscosity of which has been increased, is discharged out of the ink jet printing head in advance of printing, thus preventing printing image failures from occurring.
In the case that the printing liquid further evaporates and viscosity of the printing liquid further increases, measures are generally taken also, in which the printing liquid is sucked through the cap from the ejection ports or the printing liquid is pressed at an ink supply system to the ink jet printing head, thereby forcing the printing liquid in the nozzles out and then, instead, sending a fresh printing liquid (hereinafter “suction” and “pressurization” are collectively referred to as a “recovery operation”).
Further, a supply arrangement is available for supplying the printing head with the printing liquid, in which a printing liquid reservoir reserving therein the printing liquid is installed in the printing apparatus and the printing liquid is supplied from the printing liquid reservoir to the printing head through a supply tube or the like. In this arrangement, when the printing liquid contained in one printing liquid reservoir runs out, that particular printing liquid reservoir is replaced with a new one to permit continued use of the printing liquid. At this time of the replacement of the empty printing liquid reservoir with a new one, the recovery operation described in the foregoing is performed in order to fill a supply path between the printing liquid reservoir and the printing head with the printing liquid. The printing liquid in the supply path evaporates and bubbles are formed and accumulated in the supply path even while the printing liquid in the printing liquid reservoir is being consumed. This also necessitates the recovery operation. The longer the length of the supply path and the smaller the cross-sectional area thereof, the more conspicuous this phenomenon becomes.
With the recent trend in the printing apparatus where the printing apparatus becomes more and more compact, a cartridge type ink jet head having a printing head portion integrated with a printing liquid reservoir portion, and an arrangement allowing the printing liquid reservoir and the printing head to be replaced with a new one independently of each other are spread. Each of these arrangements requires a shorter supply path extending from the printing liquid reservoir to the printing head, reducing an effect from bubbles formed in the supply path on ejection failures. If an arrangement allows only the printing liquid reservoir to be replaced, however, it becomes necessary to perform the recovery operation for filling the supply path with the printing liquid.
FIGS. 1A to 1C are schematic cross-sectional views for explaining the operation of the cap during a suction recovery operation. FIG. 1A shows a capping state of time when, after a cap 103 is brought into tight contact with an ejection port surface 102 of a printing head 101 and a suction pump (not shown) connected to the cap 103 is caused to generate a negative pressure for sucking ink from the ejection ports, the inside of the cap is almost released from the negative pressure (or the capping state of time when the negative pressure has decreased to the extent that does not destroy a meniscus of the ejection port). Shaded portions 104 in FIGS. 1A to 1C represent ink that has been sucked out. In the condition shown in FIG. 1A, an interior of the cap 103 is considered to be substantially filled with ink.
When an attempt is made to pull apart the cap 103 from the ejection port surface in the capping state shown in FIG. 1A, a force sucking up ink, which is caused by an adhesion of the ink and the negative pressure in the ejection ports, acts at an interface between the ejection port surface 102 and the ink 104. In addition, a surface tension, that acts the ink itself to adhere, also acts on the ink inside the cap 103. These forces cause constrictions 105 to be formed in the ink 104. As the cap 103 leaves off the ejection port surface 102, a cross-sectional area of a portion of each of the constrictions 105 in the ink 104 becomes small and the weakest. The cohesion of the ink 104 then eventually is broken at the portion of each constriction 105.
FIG. 1C shows a condition, in which the ink has been broken at each of the shoulders 105. After the ink has been broken, a part of the ink is left on the ejection port surface 102 as shown in FIG. 1C. The amount of ink left on the ejection port surface 102 in this case is more than the amount of ink depositions produced from a mist and the like that is caused during a printing operation, and has tendency to become more when the lower the surface tension of the ink and the lower a water repellency of the ejection port surface 102 become. Also, the more the amount of ink depositions on the ejection port surface 102 is, the greater a load on a wiping blade and a wiper cleaner is, thus shorting a service life thereof. In operating examples shown in FIGS. 1A to 1C, the ink left inside the cap 103 when the cap 103 is released presents another problem of the ink drooping or scattering.
Furthermore, if the cap 103 is separated immediately after the ink has been sucked, an atmospheric pressure is applied instantaneously to the inside of the cap, in which there is left the negative pressure. As a result, sudden fluctuations in pressure and a mechanical impact exerted when the cap is separated destroy the meniscus inside the ejection port, causing air to get into the back of the ejection ports. This results in an ink ejection failure occurring. An arrangement as described in Japanese Patent Application Laid-open No. 2001-58421 is known as a solution to such a problem. According to the arrangement as described in this publication, after the ink has been sucked, the cap is opened after the negative pressure generated in the cap disappears.
There is known another arrangement for solving the problem described in the foregoing, in which an atmospheric air communicating valve is provided as a path allowing an atmosphere air into the cap. According to this arrangement, the atmospheric air communicating valve allows the atmosphere air to be drawn in with the negative pressure generated in the cap kept in a condition of the printing head and the cap being in tight contact with each other. This makes it possible to suppress the amount of ink left on the ejection port surface after the suction of ink and prevent the atmospheric pressure from being instantaneously applied to the inside of the cap.
The ink jet printing apparatus as described in the foregoing nonetheless has the following problems to be solved.
According to the arrangement as described in the aforementioned publication, ink of a mixture of a plurality of different colors stays inside the cap for an extended period of time and can sometimes flow back to the printing head. If an image is printed in this condition, an image problem of a color mixture is likely to occur, in which the original colors are mixed with other ones. If the cap is opened after the negative pressure generated in the cap disappears, after the ink suction, more ink is wastefully discharged through suction, thus presenting another problem of an increasing running cost.
In the arrangement using the atmospheric air communicating valve, there is a problem of difficulty in providing the valve particularly in a small size apparatus. More specifically, the recent trend in ink jet printing apparatuses is toward a smaller size body, which makes it also necessary to make smaller the cap and surrounding mechanisms. As a result, it has become relatively difficult to provide a small cap with the atmospheric air communicating valve or dispose a mechanism for operating the atmospheric air communicating valve. Furthermore, there may be cases, in which a tube for connecting the atmospheric air communicating valve is plugged with dust and dirt and becomes inoperative. Still another problem is that the apparatus gets more complicated in construction or costly, since the atmospheric air communicating valve requires a driving source for the exclusive use thereof or driving from another driving source.